3. Hewlett-Packard

Thirty percent of the top 50 U.S. technology companies pinpointed the PC maker as a threat in their annual filings with the SEC. CA and Oracle were among the 15 companies that listed HP in the risk factors or competitors section of their 10-K reports.

As part of HP CEO Meg Whitman's turnaround, the company has decided to hang onto its PC business while developing new technology for computer servers.